1. Field of the Invention
The present invention relates to an exhaust gas purification apparatus, and more particularly, to an apparatus for purifying an exhaust gas discharged from an internal combustion engine used in an automobile or the like.
2. Description of the Related Art
For the purpose of purifying an exhaust gas of an automobile, there exist an exhaust gas purifying method and an exhaust gas purification apparatus, which employ an adsorbent carrying noble metal (platinum, rhodium or the like) as a catalyst. In the above-described method and apparatus, it is generally necessary that the adsorbent be at a temperature equal to or higher than a catalyst activating temperature of 350.degree. C. in order to purify HC in the exhaust gas.
However, the conventional art has a problem in that HC is hardly purified immediately after the start of the engine because the temperature of the aforesaid catalyst does not reach the catalyst activating temperature.
To solve this problem, there has been proposed a purification apparatus including a catalyst unit provided in an exhaust system of an engine and an HC trapper containing an adsorbent for adsorbing HC (which will be referred to as cold HC) discharged when the engine is in a cool state, which HC trapper is disposed on the upstream or downstream side of the catalyst unit (Japanese Patent Unexamined Publication Nos. 2-135126, 4-17710 and 4-311618).
The purification apparatus disclosed in Japanese Patent Unexamined Publication No. 2-135126 employs an adsorbent unit and a catalyst unit in combination, which adsorbent unit using a zeolitic adsorbent is located on the upstream side of the catalyst unit. In this apparatus, cold HC is adsorbed by the adsorbent when an exhaust gas is at a low temperature and the HC desorbed from the adsorbent and HC discharged from the engine are purified by the catalyst unit when the exhaust gas is at a high temperature.
In each of the purification apparatuses disclosed in Japanese Patent Unexamined Publication Nos. 4-17710 and 4-311618, an HC trapper containing an absorbent is provided on the downstream side of a catalyst unit in parallel with a main exhaust pipe, and flow-path change-over valves are provided respectively in a bypass pipe including the trapper and the main exhaust pipe. These valves are operated for a certain period of time immediately after the start of an engine so that an exhaust gas flows through the bypass pipe, during which cold HC is adsorbed by the trapper. Upon a lapse of the certain period of time after the start of the engine, during which the temperature of the exhaust gas rises and the cold HC is brought into a state in which it is desorbed from the adsorbent of the HC trapper, the valves are turned to positions for flowing the exhaust gas into the main exhaust pipe. At this time, a negative pressure in an intake pipe of the engine is applied to a desorption pipe connecting the downstream side of the trapper to the intake pipe of the engine, so that the desorbed HC is sucked into the intake pipe so as to be burned again in the engine.
Further, Japanese Patent Unexamined Publication No. 4-311618 discloses an example in which desorbed HC is sucked to return to the upstream side of a catalyst by means of a suction pump.
Among the above-described conventional techniques for adsorbing cold HC, the technique including the HC trapper provided on the upstream side of the catalyst unit has a problem in that the adsorbing capacity of the adsorbent is impaired by the high-temperature exhaust gas which flows in the HC trapper immediately after it is discharged from the engine. In view of the above, the apparatus disclosed in Japanese Patent Unexamined Publication No. 2-135126 uses the zeolitic type adsorbent having a high heat resistance. However, the adsorbing capacity of the adsorbent is generally higher as it is at a lower temperature. Even the zeolitic type adsorbent desorbs HC before the catalyst reaches an activating temperature. Accordingly, the adsorbed HC is discharged to the atmosphere without being purified. Further, when the HC trapper is provided upstream of the catalyst unit, since the trapper itself possesses a large heat capacity, there arises a problem in that a time required for activating the catalyst or a time required until the catalyst reaches an activating temperature is delayed.
On the other hand, the apparatus disclosed in Japanese Patent Unexamined Publication No. 4-17710 or 4-311618 in which the HC trapper is provided downstream of the catalyst unit, solves the above-described problems in respect of the cold HC adsorbing capacity and the catalyst activation. However, the heat of the exhaust gas is conducted to the adsorbent slowly through the bypass pipe and it takes a long time to desorb the HC from the absorbent, so that the desorption is not always completed. If this unfavorable state is repeated, HC is accumulated in the adsorbent, and the adsorbent is saturated with too much HC to adsorb the same. Further, when the exhaust gas containing HC is returned to the intake pipe, an error occurs in the air-fuel ratio control of the engine, so that an adverse effect may be caused on fuel consumption, exhaust gas purification or drivability.
According to the apparatus of the type in which the desorbed HC is sucked to return to the upstream side of the catalyst by means of the suction pump, the aforesaid problems can be solved because an error hardly occurs in the air-fuel ratio control of the engine. However, it is difficult and expensive to develop a suction pump which can withstand severe exhaust-gaseous conditions (high-temperature, high-humidity and highly-corrosive atmosphere) for a long period of time. Therefore, this apparatus cannot be easily realized and it conclusively lacks mass productivity.
In view of the above, the inventors of the present invention propose in Japanese Patent Application No. 5-61036 an exhaust gas purification apparatus which can solve almost all the above-described problems. More specifically, in the apparatus of Japanese Patent Application No. 5-61036, an HC trapper is provided on the downstream side of a catalyst unit, and the HC trapper is provided in one exhaust pipe and an interior of the trapper is partitioned in place of providing a bypass pipe. With such structure, the heat of the exhaust gas is preferably conducted to the adsorbent so as to promote the desorption of HC.
Also, there is provided a recirculation flow pipe including a one-way valve between the upstream side of the catalyst unit and the HC trapper. The desorbed HC is returned to the upstream side of the catalyst unit by the pulsation of the exhaust gas through the recirculation flow pipe, and the returned HC is purified by the catalyst.
As a result of further investigations by the inventors of this invention after the above application, it has become apparent that the flow rate of recirculation through the flow pipe is increased or decreased depending on the position of provision of the one-way valve within the recirculation flow pipe which valve receives the exhaust gas pulsation. It has also become apparent that the flow rate of recirculation is increased or decreased depending on the position of a port of the recirculation flow pipe provided in an exhaust manifold.